Lithography involving shorter-wavelength, "deep" ultraviolet ("DUV") radiation (less than 300 nm), as well as lithographic processes involving electron-beam (e-beam) and x-ray exposure techniques, have become indispensable to the microelectronics industry. Improvement of these systems continues to be of the utmost importance.
In use, a radiation-sensitive lithographic photoresist composition is applied as a thin coating on a substrate (i.e., a wafer), and the coated substrate is subsequently "baked" to remove the casting solvent. The photoresist film is then exposed to radiation in an imagewise fashion (through a mask, with DUV and x-ray lithography, or directly, with e-beam lithography), and the image is then developed, typically by immersion in a developer solvent. With a positive resist composition, the developer removes exposed areas of the composition while leaving unexposed areas intact. With a negative resist composition, the developer removes unexposed areas of the composition while leaving exposed areas intact. Thus, for the process to be effective, there must be a differential between the solubility of the exposed photoresist and the solubility of the unexposed photoresist in the developer solvent. Indeed, one of the ways in which the industry has attempted to improve lithographic photoresist compositions has been to modify such compositions, in one fashion or another, with regard to solubility characteristics.
For example, U.S. Pat. No. 4,491,628 to Ito et al., assigned to IBM Corporation, describes deep UV photoresist compositions containing a polymer bearing acid-labile pendant groups, e.g., t-butyl esters of carboxylic acids, t-butyl carbonates of phenols, and the like. The solubility characteristics of the polymers change markedly upon acidolysis, i.e., upon acid cleavage of the pendant groups. When the resist is imagewise exposed to radiation, the photoinitiator contained in the exposed portions of the resist generates an acid, and the polymer undergoes cleavage. In the unexposed portions of the resist, the polymer remains intact. The intact polymer in the unexposed portions of the resist is soluble in a nonpolar solvent but insoluble in an alkaline or polar solvent, while the acid-cleaved polymeric product in the exposed portions of the resist is, by contrast, insoluble in a nonpolar solvent but soluble in an alkaline or polar solvent. The differential in solubility between the intact polymer and the acid-cleaved polymeric product serves as the basis for developing the image, and can be used to enable image reversal depending on the polarity of the solvent used to develop the image.
Following this initial work with homopolymers, lithographic photoresist compositions were next prepared using copolymers, e.g., copolymers formed from acrylate, methacrylate and/or styrene monomers, and a technique known as "chemical amplification." With chemical amplification, a single photochemical event essentially triggers a cascade of subsequent chemical transformations, resulting from the catalytic activity of the initially generated species. A particular copolymer of interest is described by Ito et al. (1994), in "Environmentally Stable Chemical Amplification Positive Resist: Principle, Chemistry, Contamination Resistance and Lithographic Feasibility," J. Photopolym. Sci. and Tech. 7(3):433-448, which pertains to a positive resist composition designated as an "Environmentally Stable Chemical Amplification Positive Resist" or an "ESCAP." The polymer in the disclosed resist composition is a copolymer of 4-hydroxystyrene and t-butyl acrylate. As in the '628 patent discussed above, the copolymer of the resist composition undergoes marked changes in solubility characteristics upon removal of acid-labile protecting groups.
U.S. Pat. No. 5,625,020 to Breyta et al., assigned to IBM Corporation, also describes a chemically amplified photosensitive resist composition. As in Ito et al. (1994), this patent emphasizes the use of a copolymer in the resist composition, which, depending on the presence or absence of certain pendant groups, is either soluble or insoluble in the selected developer. The disclosed polymers are copolymers of hydroxystyrene and either acrylate monomers, methacrylate monomers, or a mixture of acrylate and methacrylate monomers.
The use of dissolution inhibitors in lithographic photoresist compositions has also been proposed, and, indeed, acid-sensitive dissolution inhibitors are often incorporated into current, commercially available photoresists. With positive resists, the dissolution inhibitors are compounds that inhibit dissolution of the resist under basic conditions but allow and in fact facilitate dissolution under acidic conditions, i.e., after exposure to radiation.
Dissolution inhibitors that have been used, to date, include lower alkyl esters of cholic, ursocholic and lithocholic acid, including methyl cholate, methyl lithocholate, methyl ursocholate, t-butyl cholate, t-butyl lithocholate, t-butyl ursocholate, and the like (see, e.g., Allen et al. (1995), "Resolution and Etch Resistance of a Family of 193 nm Positive Resists," J. Photopolym. Sci. and Tech. 8(4):623-636); hydroxyl-substituted analogs of such compounds (ibid.); and androstane-17-alkylcarboxylates substituted with 1 to 3 C.sub.1 -C.sub.4 fluoroalkyl carbonyloxy substituents, such as t-butyl trifluoroacetyllithocholate (see, e.g., U.S. Pat. No. 5,580,694 to Allen et al.). Still other acid-labile dissolution inhibitors are shown below: ##STR1##
The currently available dissolution inhibitors are, however, relatively weak in their dissolution inhibition effects and do not provide optimum developer selectivity or development contrast. The present invention is directed to a new class of dissolution inhibitors which overcome the aforementioned disadvantages in the art. The dissolution inhibitors now provided are in the calixarene family, and, more particularly, are calix[4]resorcinarenes that are partially or wholly protected, preferably with acid-labile functionalities.
Calixarenes were discovered and methods of synthesis devised therefor in the 1950's. See, for example, Hayes et al. (1956) Chem. Ind. (Berlin), at p. 193 and Hayes et al. (1958) J. Appl. Chem. 8:743. Initial work with these compounds focused on potential biological applications, particularly in constructing systems for mimicking the catalytic activity of certain enzymes. Gutsche (1983), "Calixarenes," Acc. Chem. Res. 16:161-170. Other work has suggested utility as synthetic analogues of cyclodextrins, and in extraction and separation processes. Arduini et al. (1984), "p-t-Butyl-Calix[4]arene Tetracarboxylic Acid. A Water Soluble Calixarene in a Cone Structure," J. Chem. Soc., Chem. Commun., at pp. 981-982. The only references pertaining to potential use of calixarene-type compounds in lithography are Fujita et al. (1996), "Ultrahigh Resolution of Calixarene Negative Resist in Electron Beam Lithography," Appl. Phys. Lett. 68(9):1297-1299, Ohnishi et al. (1997), "Calixarene Resists for Nano-Lithography," Proc. ACS Div. Polym. Mater. Sci. Eng. 77: 453-454, Nakayama et al. (1997), "A Negative-Working Alkaline Developable Photoresist Based on Calix[4]resorcinarene, a Cross-linker, and a Photoacid Generator," Chem. Lett., at pp. 265-266, and Ueda et al. (1997), "A Negative-Working Alkaline Developable Photoresist Based on Calix[4]resorcinarenes, a Crosslinker, and a Photoacid Generator," Proc. ACS Div. Polym. Mater. Sci. Eng. 77: 455-456. However, each of these publications describes cross-linked calixarenes as the polymeric component of the photoresist, and fails to recognize the potential utility of a calixarene compound as a dissolution inhibitor. Furthermore, none of these references suggest that calixarenes which are partially or wholly protected, either with acid-labile protecting groups or other moieties, could be desirable for any purpose.
Accordingly, the present invention is directed to the discovery that certain compounds in the calixarene family are excellent dissolution inhibitors in a lithographic photoresist composition. The invention represents an important advance in the art, insofar as developer selectivity is greatly increased and substantially higher dissolution inhibition is provided. These features in turn give rise to high development contrast and utility at relatively low concentrations. The compounds can also be used in conjunction with a variety of polymers, resist components, and customary optional additives, as will be discussed in detail herein. As noted above, the present dissolution inhibitors are in the calixarene family, and, more particularly, are calix[4]resorcinarenes that are partially or wholly protected, preferably with acid-labile functionalities. That is, calix[4]resorcinarenes are cyclic tetramers of resorcinol and possess eight phenolic OH groups, and in the compounds of the invention at least one of the phenolic OH groups is protected, preferably with an acid-labile protecting group to give an acid-labile dissolution inhibitor. Photochemically induced acid-catalyzed deprotection regenerates the unprotected calix[4]resorcinarene, which dissolves extremely rapidly in aqueous base.